In an oil and gas well, one or more strings of casing will be cemented within the well. In one system used with offshore jack-up drilling rigs, a mudline hanger located in a subsea housing at the sea floor will support the string of casing in the well. A section of the casing will extend upward to a surface wellhead housing on the drill rig. The surface wellhead housing will be located above the sea and below the rig floor. The distance from the subsea housing to the surface wellhead could be as much as 500 feet with a large jack-up drilling rig.
Cement will be pumped down the string to flow up the annulus to cement the casing in the well. The level of cement will be below the mudline hanger. The casing will be cut off at the surface wellhead. The blowout preventer will be removed, and a spear will be used to pull tension on the casing after cementing. Then slips will be inserted around the casing, which engage the wellhead housing and grip the casing to hold the casing in tension. A packoff will be installed between the casing hanger and the wellhead housing.
A disadvantage of this system is that the blowout preventer must be removed while installing the slips and packoff. A danger of a blowout thus exists. Also, this system is time consuming and expensive. In addition to this, the sealing mechanisms are generally elastomer or on site machined to give metal-to-metal seals.
In another design, described in U.S. Pat. No. 5,002,131 after cementing, dogs mounted to the exterior of the casing hanger are released. Each of the dogs has a set of circumferential grooves or wickers on the exterior for engaging the wellhead housing. The wellhead housing has a mating set of grooves or wickers. Springs urge the dogs outward.
The running tool for the casing hanger has a sleeve retainer. This retainer holds the dogs in the retracted position during cementing. After cementing, rotating the running tool unscrews the running tool from the casing hanger. When this occurs, the sleeve moves upward, releasing the dogs to engage the wellhead housing.
Before the running tool completely releases, tension is applied to the casing to the desired amount. The dogs ratchet over the wickers in the wellhead housing as the casing hanger moves up while the tension is applied. The dogs grip the wellhead housing, preventing the casing hanger from moving downward. The running tool and sleeve are then removed from the wellhead housing. Thereafter, in a separate trip, a seal assembly is installed to seal the annular gap between the string and the wellhead. A similar design is disclosed in U.S. Pat. No. 5,255,746. String tensioning devices are generally illustrated in U.S. Pat. Nos. 5,310,007 and 5,839,512.
The present invention seeks to overcome some of the shortcomings of the prior designs. It provides a one-trip method to apply tension to the string and to seal the hanger in the annular space. It also has capability to lock the hanger in a sealed position in the same single trip. It accordingly minimizes the time the annular space is open and improves the safety of the operation by providing the isolation capability in that same single trip. These and other advantages of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment and the claims that appear below.